The chloroform extract of mustard leaves (Brassica juncea Cosson) reduced mutagenicity of AFB₁ in bacterial assay (Salmonella typhimurium TA100). 4-Decanol was one of major compounds in the chloroform extract when analyzed by GC-MS on HP-5 capillary column. The authentic compound of 4-decanol dissolved in DMSO (0.5%) inhibited mutagenic activities of AFB₁ and MNNG in Salmonella typhimurium TA100 at a rate of 99% and 93%, respectively. This result indicates that 4-decanol is an antimutagenic compound present in chloroform extract of mustard leaves.